The optical efficiency of a light emitting diode (LED) is greatly influenced by the difference between and the matching of the refractive indexes of the light emitting diode chip and of a functional optical element applied thereto, for example, a phosphor layer or a lens. The connection between the light emitting diode chip, which comprises a high refractive index semiconductor material having a refractive index in the range of 2.4 to 3.5, and, for example, a ceramic phosphor layer having a refractive index in the range of 1.8 to 2.8 or a glass lens having a refractive index in the range of 1.46 to 2.1 is usually produced by a low refractive index silicone layer having a refractive index in the range of 1.4 to 1.55, which leads to significant optical losses of approximately 5% to 10%. In order to increase the coupling-out efficiency of an LED and in order, in particular, to be able to advance to efficiencies of more than 150 lumens per watt, it is necessary to minimize such optical losses as much as possible.
The endeavor to couple optical elements to a light emitting diode chip as far as possible without layers having a low refractive index is known. By way of example, attempts are made to use conventional adhesive layers having the highest possible refractive index. However, since there are no optical adhesives exhibiting long-term stability and having refractive indexes of more than 1.55, this approach has been limited hitherto. Furthermore, attempts are made to apply an optical element without adhesion layers on a light emitting diode chip by means of direct bonding. Although direct bonding would constitute the best connection optically and thermally, in practice it is useable only to a very limited extent on account of the stringent requirements of the surface quality. Furthermore, the direct deposition of semiconductor materials on an optical element is also discussed. However, the direct deposition of epitaxial material on optically active substances has not yet been sufficiently demonstrated hitherto and furthermore very greatly restricts the choice of the suitable materials to be used.